Outside Doctor Who, Graham is perhaps best known for providing the voice of "Brains" in the popular "Supermarionation" adventure series, Thunderbirds. He recently reprised that voice for a series of credit card commercials in New Zealand. He reprised another Thunderbirds character, Parker for the 2015 reboot, Thunderbirds Are Go!.

Graham was also the "Big Brother" character on the giant screen for the 1984 television commercial debuting the Apple Macintosh computer. (David's official website)